The present application incorporates by reference U.S. Pat. No. 5,654,755, assigned to the assignee hereof.
The present invention relates to image sensor arrays used in raster input scanners. In particular, the invention relates to photosensitive chips wherein each photosensor outputs signals onto a common video line, and where there are provided dark photosensors for setting an offset level on the common video line.
Image sensor arrays typically comprise a linear array of photosensors which raster scan an image bearing document and convert the microscopic image areas viewed by each photosensor to image signal charges. Following an integration period, the image signal charges are amplified and transferred as an analog video signal to a common output line or bus through successively actuated multiplexing transistors.
For high-performance image sensor arrays, a preferred design includes an array of photosensors of a width comparable to the width of a page being scanned, to permit one-to-one imaging without reductive optics. In order to provide such a xe2x80x9cfull-widthxe2x80x9d array, relatively large silicon structures must be used to define the large number of photosensors. A preferred technique to create such a large array is to make the array out of several butted silicon chips. In one proposed design, an array is intended to be made of 20 silicon chips, butted end-to-end, each chip having 248 active photosensors spaced at 400 photosensors per inch.
Although most scanning systems currently in use are ultimately digital systems, the xe2x80x9craw signalxe2x80x9d coming out of the photosensors during the scanning process is an analog video signal, with the voltage magnitude corresponding to the intensity of light impinging on the photosensor at a given time. Thus, when signals are read out from the photosensors on a chip to be converted to digital data, different video levels, corresponding to the brightness of the reflected area being scanned by a particular photosensor at a particular moment, are output as a series of analog voltage levels.
Photosensitive devices may be one-dimensional or two-dimensional, and can be either of the xe2x80x9cactivexe2x80x9d variety, wherein the photosensors output voltage signals, or in the form of a charge-coupled device, or CCD, which outputs a sequence of charges from a series of individual photosensors. In all of these various types of photosensitive devices, a common design feature is the use of xe2x80x9cdarkxe2x80x9d photosensors, which are used to periodically reset the offset voltage for the photosensors being read out. These dark photosensors are of the same semiconductor structure as the other xe2x80x9cactivexe2x80x9d photosensors on each chip, but the dark photosensors are not exposed to light. In most designs, the dark photosensors are provided with an opaque shield, such as of aluminum or silicon, to prevent the influence of light thereon. In the scanning process, with each readout cycle of active photosensors on each chip, the readout of the first photosensor is proceeded by readouts of one or more dark photosensors, which are used to reset the voltage offset associated with the whole chip, and thereby correct signal drift when the active photosensors are reading out their signals. In other words, the readout of a dark photosensor with each scan can serve as a reference offset or xe2x80x9czero pointxe2x80x9d so that the absolute values of light intensity on the active photosensors may be determined. The use of a dark photosensor output when reading out signals from active photosensors can significantly compensate for performance variations of multiple chips in a single apparatus, and also for changes in the performance of a photosensitive device over time.
U.S. Pat. No. 5,654,755 describes a circuit for correcting the offset of the video output of a set of active photosensors, based on the output of dark photosensors. An averaging RC circuit in parallel with the video line accumulates an average signal based on a large number of readings from the dark photosensors. The average signal is periodically clamped to a correction capacitor in series on the video line. The charged correction capacitor adjusts the offset on the active-photosensor signals which subsequently pass through the video line. In this context, the correction of the offset on active-photosensor signals is known as xe2x80x9cDC restore.xe2x80x9d
While the system of the ""755 patent works well from the perspective of correcting offset on an integrated photosensor chip, certain subtleties of operation must be addressed when such a chip is incorporated into a larger system. One problem is that the signals from the dark photosensors add to the fixed-pattern noise, or dark non-uniformity, of the video signals that must be processed. Dark photosensors should have the same drift characteristics as the active photosensors. In the case where dark photosensor signals are flushed straight through the video circuitry, the on-chip drift characteristic follows that of the drift of the photosensor circuitry in addition to the drift of the video amplifiers. However, the drift of the active photosensors does not reflect the drift of the circuitry since this is subtracted out during the DC restore operation. During the DC restore operation the video signal is restored to a dark reference level plus the active pixel level minus the averaged dark photosensor level. Since both the active photosensor level and the dark photosensor level have the same drift, this drift is cancelled out in the video signal. Therefore there exists a need to provide an offset-correction system for dark and active photosensors, which provides the offset correction from both the perspective of the on-chip photosensor circuitry, and also from the perspective of any downstream image circuitry.
According to the present invention, there is provided a photosensitive device and a method of operating thereof. The photosensitive device comprises a set of photosensors, each photosensor outputting a voltage signal representative of light intensity thereon. A video line is adapted to receive voltage signals from the set of photosensors. A correction capacitor is associated with the video line, the correction capacitor adapted to retain a correction charge thereon to influence the voltage signals from the photosensors. A bypass switch selectably causes the signal on the video line to bypass the correction capacitor. A signal is read from a photosensor a first time with the correction capacitor bypassed by the bypass switch and then a signal is read from the photosensor a second time through the correction capacitor.
According to another aspect of the present invention, there is provided a photosensitive device and method of operating thereof. The photosensitive device comprises a set of photosensors, each photosensor outputting a voltage signal representative of light intensity thereon. A video line is adapted to receive voltage signals from the set of photosensors. An averaging circuit is in parallel with the video line. Within a cycle of operation, a signal is read from a photosensor a first time, with reading a signal to the averaging circuit, and then a signal is read from the photosensor a second time, but the signal is not read to the averaging circuit.